catch (one) in the act

To see, and perhaps apprehend, someone as they are doing something (often something nefarious). The police caught the robbers in the act of ransacking another house. I suspected that my friend was stealing money from me, and then I caught her in the act. Ah, our doorbell camera caught someone in the act! Look at this guy stealing our packages!

caught in the act

Seen, and perhaps apprehended, while doing something (often something nefarious). Our security cameras caught the burglar in the act. Those robbers were finally arrested when they were caught in the act of ransacking another house. Alyssa, how do you always manage to sneak out and go to parties? I'm always caught in the act by my mom before I get a toe outside the house.

caught in the act

and caught red-handed

Fig. seen doing something illegal or private. (See also catch someone in the act (of doing something) and catch someone red-handed.) Tom was caught in the act and cannot deny what he did. Many car thieves are caught red-handed.